(1) Field of the Invention
This invention relates to a method and apparatus for detecting and locating faults in analogue systems from the response of those systems to applied stimuli. The method is novel in the way in which it represents parameters within the system and models the expected behaviour of the system and its component parts.
The equipment comprises a means of interfacing with the analogue system to be tested, or Unit Under Test (UUT). This interface is as a standard piece of Automatic Test Equipment and provides power and stimuli as inputs to the UUT, and a load for the outputs. It also provides a means for measuring the values of the inputs and outputs, and also for taking measurements of other points within the UUT (Probing). The equipment also comprises a means for making physical and electrical contact to the UUT for the purposes of the test.
The power supplied and the stimuli are so chosen that the UUT can operate in the conditions under which it is to be tested. The outputs are created from the input stimuli of the UUT transformed by its function. The stimuli are also so chosen that, should a fault be introduced into the UUT, the output will be different from the output in a working state. The values of the inputs and outputs are all measured by the equipment and provided to computing equipment for analysis.
(2) Description of the Prior Art
Both In Circuit Testing (ICT) and Functional Testing are used for analogue as well as digital circuits. However, the problems of detecting and locating faults in analogue circuits are different and in some ways greater than for digital circuits. See for example P. Deves, P. Dague, J. Marx, O. Raiman Dedale: An Expert System for Troubleshooting Analogue Circuits, 1987 International Test Conference Proceedings pp 586-594. ICT attempts to isolate components and test them individually. Its needs for access to the components' connections is leading to increasing difficulties as packing densities increase. ICT forces conditions on the components in an attempt to isolate them from their environment, and then measures their response. The test generation is relatively easy because the circuit is tested a component at a time. However, there is often concern about the stresses caused by back-driving components on the board, and the function of the complete circuit is not tested. Functional Testing is often preferred for these reasons.
Functional Testing of a circuit is done by making it exercise its designed functions as accurately as possible. Consequently functional test programms are much more difficult to write, and the location of faults becomes much more complex. In digital circuits, fault location algorithms exist and fall into two categories, Fault Dictionary and Deductive.
Fault Dictionaries contain the responses expected from faults which can be simulated. In the case of digital circuits, the "stuck at" fault models provide an approximation to faults in both the components and connections.
In analogue circuits there are many possible modes of failure and consequent effects and the effects of tolerances make the definition of expected measurements imprecise. This makes Fault Dictionaries inappropriate. Other techniques for fault location often rely upon a representation of cause and effect in directed graphs. Analogue circuits tend to comprise closed loop feedback paths and large numbers of components which, like resistors, have no particular input or output. This makes directed graphs inappropriate.
There is no single set of values which the measurements must take for an analogue circuit to be said to be working. Acceptable test results have ranges which are dependent upon the tolerances within the circuit as well as the accuracy of the measurements. Any successful automatic diagnosis technique must take this into account. Also, it is difficult to measure the current flowing between components, but this is often a vital piece of information to define the state of a circuit.
Because of these difficulties most diagnosis of faults in analogue circuits has to be done by skilled technicians. This invention makes it possible to detect and locate faults in analogue circuits using minimal probing and only voltage measurements (although this does not mean to say that current measurements may not be used if desired). The method developed uses only descriptions of correct behaviour so that, provided the description is appropriately constrained, any type of fault is detectable.